A ball is moving on a perfectly smooth horizontal surface. If no force is applied on it, will it's speed decrease, increase or remain unchanged?

It's speed will remain unchanged.

Reason — According to Newton's first law of motion, if a body is in a state of rest, it will remain in the state of rest and if it is in the state of motion, it will remain moving in the same direction with the same speed unless an external force is applied on it.